The present invention relates to a touch responsive key arrangement for a keyboard controlled electronic musical instrument. In a preferred embodiment, the keyboard is responsive to depression velocity and aftertouch pressure.
In most prior art electronic keyboard musical instruments, such as electronic organs, synthesizers and electronic pianos, the depression of the playing keys closes a switch, and the switch closure is interpreted by the electronic circuitry to produce tones having frequencies corresponding to the depressed keys. In a velocity sensitive electronic piano, the amount of force or speed with which a key is depressed controls the amplitude of the resultant tone to thereby simulate the operation of an acoustic piano. In many prior art electronic pianos, a contact member is disconnected from one bus and then brought into contact with a second bus on full key depression, and the time of travel of the contact between the two buses is measured and the amplitude of the tone is altered in accordance therewith.
In many systems, the timing is accomplished by charging or discharging capacitors assigned to the respective keys. Alternatively, the number of discrete time elements, such as scans of the keyboard, that occur while the movable contact is moving between the two buses is detected. The higher the velocity with which the key is depressed, the shorter will be the time of transit between the two buses.
Some prior art keyboards include what is known as aftertouch control wherein further depression of the key after it has reached its normally depressed position alters the quality of the tone. The aftertouch sensing can be accomplished by a piezoelectric element contacted by the key or a lever attached to the key, by compressing a conductive compressible strip to alter the resistance thereof, or by altering the area of contact between a stationary contact member and a deformable contact member.
A disadvantage to keyboards that utilize actual switch closures to key the tones is that the switch contacts become dirty after a period of time thereby causing scratch and intermittent operation. Furthermore, there is the problem of switch bounce, which is particularly troublesome in digital instruments because the digital circuitry detects the switch bounce as repeated key closures whereas only one key closure is intended. Bounce must be overcome by utilizing switch debouncing circuitry.
Switch contacts can also become deformed so that different switches will close at different points in the depression of their respective keys. In velocity sensitive keyboards, such as those used in electronic pianos, inconsistencies in switch closure times will result in some keys producing tones of different amplitudes for a given key depression velocity, thereby resulting in non-uniform operation.
Uniform and predictable aftertouch control is particularly difficult to achieve when utilizing compressible resistive strips or piezoelectric elements, because their electrical characteristics may vary with time and use. Furthermore, electrical changes resulting from physical changes in the elements, such as in the case of the amount of compression of a resistive strip, is difficult to predict and control with the consistency that is desirable.